The difference between hard disk drives (HDDs) and solid state disks (SSDs) derives from the very physics of magnetic media and NAND flash memory. Magnetic media can be overwritten endlessly; flash memory cannot. With an HDD, the action of “deleting” a file typically affects only the metadata in the directory, which is changed to designate the affected sectors as now containing “free space” for writing new data over the old. With NAND flash memory, by contrast, free space can only be created by actually deleting or erasing the data that previously occupied a page of memory. The process of reclaiming pages of flash memory that no longer contain valid data is called “garbage collection.” Only when the pages, and the blocks they contain, have been reset in this fashion are they able to store new data during a write operation. In SSD parlance, the act of writing, then deleting data is referred to as a program/erase (P/E) cycle.